Fix late-bound requires attributes returning None (#2153)

[maxnbk]

Closes #2153

3.4.0: PR #2004 added build_requires and private_build_requires to late_bind_schemas to fix a bug in rez-search causing it to crash. However, late_requires_schema only accepted lists. So, a @late() function returning None, for example, when there are no private build requirements, would raise SchemaError: None should be instance of <class 'list'> instead of being treated as an empty list. In 3.3.0, the value passed through unchecked and the or [] in get_requires() handled it.

The fix was performed in two layers. Only one is necessary, but the second is a safety-guard for other schemas to produce warnings rather than crash.

Layer one: late_requires_schema (package_resources.py) - modified in place to accept None and normalize to [].
Layer two: _wrap_forwarded (packages.py) - The defensive check: if any late-bound attribute's schema validation fails, and the value is None, warn, and treat as unset, rather than crashing. Protects attributes beyond the requires-family (tools, cachable, etc.) that have their own schemas.

To test, a new fixture late_binding_none with @late() functions returning None for requires, build_requires, and private_build_requires. All three assert that they are normalized to [].

An additional test exercises the Layer two fix by patching the schema to intentionally hit the otherwise invalid path.

Disclosure: GLM-5.2 was used to assist in diagnostics, docstring, and test-writing.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 61.32%. Comparing base (d415b96) to head (16afd3a).
⚠️ Report is 36 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2158      +/-   ##
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+ Coverage   60.65%   61.32%   +0.66%     
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  Files         164      164              
  Lines       20584    20575       -9     
  Branches     3579     3577       -2     
==========================================
+ Hits        12485    12617     +132     
+ Misses       7224     7087     -137     
+ Partials      875      871       -4     

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[JeanChristopheMorinPerso]

@maxnbk That's an oversight from me, thanks for fixing! I never thought someone would return None in a late bound function. Do you think this is something we should discourage this practice? If so, we should add a note in the docs and also in the warning about that I think as to make it more actionable.

Thoughts?